There are two areas of cell division or MERISTEMS contributing to the growth of a stem. The apical meristem at the tip increases the length of stem and produces new leaves and buds. The other meristem is the CAMBIUM that produces new phloem and xylem. In most plants (DICOTS) this is a ring of cells that increases the girth of the stem. In plants such as lilies and grasses (MONOCOTS) the cambium is scattered through the stem and does not remain active, so they cannot increase in girth.

Diagram of long section of shoot apex and section of Coleus tip on right

Injury to the cambium layer in certain plants can kill the entire plant. For example, trees can be killed by GIRDLING damage to the cambium layer located just inside the bark.